Polypropylene has satisfactory molding processability, and molded articles obtained therefrom are excellent in mechanical properties, such as rigidity, tensile strength, flexural strength, etc., water-resistance, chemical resistance, and the like. Besides, polypropylene has a low specific gravity and is inexpensive. Hence, it has been industrially produced in large quantities for wide applications as various injection-molded articles, hollow molded articles, and extrusion-molded articles, e.g., films, sheets, tubes, fibers, tapes, etc. However, polypropylene having such advantages involves difficulty in finishing, such as coating, printing, adhesion, hot-stamping, deposition, etc., due to its non-polarity, and many attempts for overcoming this disadvantage have hitherto been made. For example, studies have been conducted on modification of a surface layer of a polypropylene molded article by physical or chemical processes prior to finishing and put into practical application. These improved processes, however, produce insufficient effects because they require complicated steps or, depending upon etching processes, accompany deformation of molded articles. Therefore, no satisfactory results can be obtained in view of increased cost.
Further, there have been proposed an improved process in which polypropylene is graft-modified with an unsaturated carboxylic acid or an anhydride thereof, e.g., acrylic acid, maleic anhydride, etc., in the presence of an initiator, such as an organic peroxide as disclosed in Japanese Patent Application (OPI) No. 76149/75 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") or Japanese Patent Publication No. 10265/76, and a process in which the above-described graft-modified polymer is blended with unmodified polypropylene. In these processes, however, degradation of polypropylene is induced by the peroxide so that the amount of the unsaturated carboxylic acid or anhydride thereof to be introduced by graft-modification is restricted. As a result, the effect on surface modification is insufficient, or if any effect may be produced, degradation is accompanied by deterioration of mechanical properties.
In order to eliminate the above-described problems, the present inventors have conducted extensive and intensive investigations. As a result, it has now been found that a large amount of a polar group can be introduced to polypropylene not only to remarkably improve finishing workability of the molded article obtained therefrom but also to improve mechanical properties of the molded article, such as impact resistance, by mixing polypropylene with an ethylene copolymer comprising an unsaturated dicarboxylic acid anhydride and an unsaturated ester as comonomers and/or a modified copolymer prepared by ring opening addition reaction of an alcohol to the anhydride group of said ethylene copolymer. The present invention has been completed based on this finding.